RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 29 
It has long been known that the blastoderm in Sauropsida 
has, in comparison with that of Ichthyopsida, this peculiarity 
that the embryonic rudiment occupies a central position from 
the beginning and never extends to the edge. This fact is 
connected with another, and as will be seen necessarily so 
connected, namely, that no part of the edge of the blastoderm 
in Sauropsida is inflected or invaginated to form the dorsal 
hypoblast. Balfour has given (‘Comp. Emb.,’ vol ii, p. 238) 
an explanation of the central position of the embryo in Saurop- 
sida. ‘To quote his own words, “The embryos in Sauropsida 
have come to occupy a central position in the blastoderm owing 
to the abbreviation of a process similar to that by which in 
Elasmobranchii the embryo is removed from the edge of the 
blastoderm ; and the primitive streak represents the linear 
streak connecting the Elasmobranch embryo with the edge of 
the blastoderm after it has become removed from its previous 
peripheral position, as well as the true neurenteric part of the 
Elasmobranch blastopore.” ‘This view is perfectly correct as 
far as it goes, but the facts need more minute analysis than 
Balfour undertakes. The primitive streak referred to by 
Balfour extends from the neurenteric canal, that is, from the 
posterior extremity of the complete embryo, some distance 
forward along the ventral median line. But this is not the 
whole of the primitive streak. There is an anterior part which 
is contained within the region enveloped by the medullary 
folds ; along the line of this part the epiblast is continuous 
with the dorsal hypoblast, and actual apertures have been 
observed through the streak by which the cavity of the com- 
mencing intestine communicates with the exterior round the 
end of the growing notochord. This part of the primitive 
streak, then, called at its earliest stage the primitive groove, 
represents the ancestral median dorsal blastopore, and is, there- 
fore, homologous with the whole of the edge of the blastoderm 
in Teleostei, and with the inflected part of the edge of the 
blastoderm in Elasmobranchii. The homology is shown in Dia- 
gram III, Pl. IV, where y z represents the final position of the 
neurenteric canal at the posterior end of the complete embryo. 
